1. Field of the Invention
The present invention is generally in the field of semiconductors. More specifically, the invention is in the field of ground shields in semiconductor dies.
2. Related Art
Accurate modeling of passive components, such as inductors, in a circuit design requires a clearly defined AC ground. This AC ground can be defined by using substrate contacts, which can be formed close to the passive component, or a metal or polysilicon ground shield situated below the passive component. The substrate contacts and ground shield can be electrically connected to a ground in an interconnect metal layer, such as interconnect metal layer one, i.e. “M1,” in a semiconductor die. For an inductor, a ground shield can cause a desirable increase in the inductor's quality factor (“Q”) while causing an undesirable increase in the inductor's capacitance, which can decrease the usable range of the inductor. By way of background, a ground shield causes a capacitance effect on an inductor that is inversely proportional to the distance between the interconnect metal layer that the inductor is fabricated in and the ground shield. In contrast to a ground shield, substrate contacts situated close to the inductor yield a lower increase in the inductor's Q while causing a less severe capacitance effect.
The use of a metal shield, which can be patterned in M1, or a polysilicon shield have been suggested to obtain an inductor having an higher quality factor (“Q”). However, a metal shield formed in M1 can severely degrade the usable frequency range or self resonance frequency (“SRF”) of an inductor by as much as 30%. Although a polysilicon shield causes only approximately one half of the SRF degradation caused by the metal shield, the Q of the inductor that is obtainable with the polysilicon shield is lower than the Q that can be realized with the metal shield.
Thus, there is a need in the art for a more effective ground shield for a passive component, such as an inductor, fabricated in a semiconductor die.